The invention relates to a method for automatic convergence correction in a television set, and to a television set having a convergence correction device.
Convergence correction devices are used for correction of parameters in the raster deflection, for example for correction of north/south and east/west distortion, pin-cushion distortion, nonlinearity in the deflection, and other geometric errors in the horizontal and vertical directions. One particular field of application is convergence correction in a projection television set, in which the pictures from three monochromatic tubes are projected onto a screen. The term television set in this context means any equipment with an electronic raster picture display. The equipment may be fed from a television broadcast signal or else, as a pure monitor, from an RGB signal, a composite video signal, or separately with a light intensity signal and a colour sub-carrier from any desired video signal source.
The deflection parameters are corrected using correction values, which are stored in a convergence circuit. The stored correction values are converted in a digital/analogue converter into an analogue control signal, and are supplied to a driver circuit, which comprises a preamplifier and an output amplifier. This driver circuit emits a current, which corresponds to the correction value, to a correction coil.
The details of such a convergence correction circuit are disclosed, for example, in Applications DE 198 01 968, DE 198 01 966 and DE 42 14 317. The convergence circuit itself is not the subject matter of the present inventions.
The convergence correction values also depend, inter alia, on the earth""s magnetic field at the point where the television set is located. This means that the convergence correction must be recorrected after the set has been transportedxe2x80x94for example from the manufacturer to the customerxe2x80x94in order to achieve an optimum setting. An entirely corresponding situation also occurs, of course, when the mechanical construction of the set changes as a result of parts being replaced for repair purposes, or as a result of external influences.
In known sets, a raster is displayed on the screen, using the three primary colours red, green and blue, in a special operating mode for convergence correction. The remote control, for example, can now be used to move the three primary colours of the raster such that they coincide, by correcting the convergence values successively for each raster crossing point. The convergence values are stored automatically in a memory provided for this purpose in the set. If, for example, there are eleven horizontal and fifteen vertical raster lines, a horizontal and a vertical correction value can thus be stored for each of the three primary colours for the 165 crossing points, that is to say a total of 990 values. This is time-consuming and tedious, not least because the optimum setting is often not found in a single attempt. Furthermore, this trimming can be carried out only by trained personnel.
Against this background, the present invention suggests a method which allows convergence correction values to be determined and to be stored considerably more easily.
The method according to the invention has the advantage that the user can easily initiate automatic convergence correction, for example by pushing a button. The method is also distinguished by the fact that it is based on a small number of optical sensors which are arranged outside the viewed area of the screen, and thus have no adverse effect on the picture. Nevertheless, convergence can be corrected over the entire screen.
According to the invention, this result is achieved by interpolation between those correction values which are determined by means of the optical sensors, which cannot be seen and are arranged at the screen edge. DE 197 00 204, for example, discloses the way in which the position of a marker is defined with respect to a sensor. This method is not the subject matter of the present inventions.
In one exemplary embodiment of the invention, the measured correction values are interpolated in the horizontal direction. The determined interpolation functions allow further convergence correction values to be calculated, which are preferably located on vertical lines of the convergence raster. The convergence values determined in this way for their part once again form support points for calculation of interpolation functions which extend in the vertical direction of the screen. The calculation of intermediate values by means of these last-mentioned interpolation functions allows convergence correction values to be defined for each crossing point of the convergence raster.
It has been found to be expedient to use polynomials whose order is limited by the number of respectively available support points as the interpolation function. Straight lines or parabolae are particularly suitable for this purpose. The advantage of these functions is that good convergence correction is achieved with little computation complexity.
In one expedient development of the method according to the invention, a fixed-position marker can be displayed in the region of the screen, which can be seen, and moveable markers in the other primary colours can be moved to coincide with the fixed-position marker. This allows the accuracy of the convergence correction to be increased. It is particularly advantageous if the fixed-position marker is displayed using that light source which allows an image with the least imaging errors, by virtue of its arrangement with respect to the screen. As a rule, this is the colour green.
In a modification of the method according to the invention, the convergence magnetic filed defined in one direction is maintained while each of the others are determined. This makes it possible to move to the required marker position more accurately. It has been found to be advantageous for the initially determined convergence magnetic field to be recorrected once again at the end, when the second convergence magnetic field is fixed. This allows increased convergence correction accuracy to be achieved.
Furthermore, the invention provides a television set, which has all the preconditions to allow the convergence correction method according to the invention and as described initially to be carried out.
One advantage of the television set according to the invention is that the optical sensors, which are required to carry out largely automatic convergence correction are arranged at the edge of the screen, where the viewer can no longer see them.
The television set is expediently also equipped with a video generator, which allows markers to be displayed using all the primary colours, and allows these to be positioned as required on the screen.
The television set is advantageously also provided with a control device which allows the convergence coils of each tube to have a current applied to them such that the actual position of a marker is shifted until this matches a required position, which is governed by the position of a sensor.
In one embodiment of the television set according to the invention, an optical sensor is arranged approximately at the centre of the screen, thus also allowing convergence correction values to be determined in the inner region of the screen.